NASA: Big asteroids not a threat to Earth

Agency has ID'd where 90% of the objects are headed

by Alicia Chang - Sept. 30, 2011 12:00 AMAssociated Press

LOS ANGELES - If you're worried about a killer asteroid wiping out Earth, NASA has some good news.

The space agency said Thursday it has identified more than 90 percent of giant near-Earth asteroids, including ones as big as the one thought to have killed the dinosaurs. None poses a danger to the planet in the next several centuries.

"We know now where most of them are and where most of them are going. That really has reduced our risk" of an impact, said Amy Mainzer of the NASA Jet Propulsion Laboratory.

NASA researchers also downgraded their estimate of the number of medium-size asteroids, saying there are 44 percent fewer than previously believed. The downside is that scientists have yet to find many mid-size asteroids, which could destroy a city.

"Fewer does not mean none," Mainzer said. "There are still tens of thousands out there that are left to find."

The updated census comes from data from NASA's sky-mapping spacecraft named Wide-field Infrared Survey Explorer, or WISE, which launched in 2009 to seek out near-Earth objects and other cosmic targets.

Unlike previous sky surveys, WISE has sensitive instruments that can pick out both dark and light objects, allowing it to get the most accurate count yet of near-Earth asteroids. The spacecraft takes a small sample of asteroids of varying sizes and then estimates how large the population would be.

For the largest asteroids - bigger than 3,300 feet across - NASA said 911 of the 981 thought to exist have been found. None poses a threat to Earth in the near future, the space agency said.

WISE data indicate there are about 19,500 between 330 and 3,300 feet wide.